FIG.1: As soon as a monitor console is added, the
microphone’s loading impedance is halved.
a monitoring console is added across the F.O.H. mixer,
microphone loading drops to 600 ohms (fig.1). When
additional mixers are added for recording and/or
broadcasting, the loading falls further, down to 300
ohms when 4 mixers’ inputs are connected. The load
impedance seen by microphones is often even lower
at high frequencies, above 5kHz, due to the capaci-
tance of many metres of multicore cable, and also
the RF filtering capacitors inside each console
How excess loading affects
Microphones.
The extra loading of multiple mixers has two effects:
First, it reduces the microphone’s output level by
Dynamic Microphone behaviour
Everyday dynamic microphones are broadly as-
sumed to have an impedance around 250 ohms. In
reality, the impedance of many widely used micro-
phones fluctuates widely at resonant points across
their frequency range, up to 2500 ohms (over 10 times
higher) and down to 50 ohms*, or one fifth of the
nominal. These impedance fluctuations aren’t a prob-
lem when microphones are plugged into a single
mixer, presenting a load that’s close to the 1200 ohms
specified for most microphones.
The trouble starts when more than one mixer needs
to be connected across each microphone. As soon as
*
eg. some models by AKG, EV, Shure and Sennheiser.
Application Notes
Application Notes
MSX 4
Active Microphone Splitter
www.arx.com.au